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HD235088 (TOI-1430) is a young star known to host a sub-Neptune-sized planetcandidate. We validated the planetary nature of HD235088 b with multibandphotometry, refined its planetary parameters, and obtained a new age estimateof the host star, placing it at 600-800 Myr. Previous spectroscopicobservations of a single transit detected an excess absorption of He Icoincident in time with the planet candidate transit. Here, we confirm thepresence of He I in the atmosphere of HD235088 b with one transit observed withCARMENES. We also detected hints of variability in the strength of the heliumsignal, with an absorption of $-$0.91$\pm$0.11%, which is slightly deeper(2$\sigma$) than the previous measurement. Furthermore, we simulated the He Isignal with a spherically symmetric 1D hydrodynamic model, finding that theupper atmosphere of HD235088 b escapes hydrodynamically with a significant massloss rate of (1.5-5) $\times$10$^{10}$g s$^{-1}$, in a relatively cold outflow,with $T$=3125$\pm$375 K, in the photon-limited escape regime. HD235088 b($R_{p}$ = 2.045$\pm$0.075 R$_{\oplus}$) is the smallest planet found to datewith a solid atmospheric detection - not just of He I but any other atom ormolecule. This positions it a benchmark planet for further analyses of evolvingyoung sub-Neptune atmospheres.

Confirmation of an He I evaporating atmosphere around the 650-Myr-old sub-Neptune HD235088 b (TOI-1430 b) with CARMENES